A number of techniques have been proposed, involving radio frequency, capacitive and optical measuring techniques, for measuring wall thickness of molded hollow glass containers after the containers have cooled--i.e., at the so-called cold end of the manufacturing process. However, it is desirable to obtain wall thickness measurement as early as possible in the manufacturing process, preferably at the so-called hot end of the manufacturing process, so that any corrective action that is needed can be implemented as quickly as possible, thereby reducing manufacture of unsatisfactory ware. It is therefore desirable to provide a technique for measuring wall thickness of molded glass containers and other similar articles as soon as possible following the molding process.
It has heretofore been recognized that glass containers that are still hot from the molding process emit radiation in the infrared range, and that this radiation can be measured in an effort to determine container wall thickness characteristics. For example, U.S. Pat. Nos. 2,915,638 and 3,356,212 propose to measure infrared energy radiated from the exterior surface of hot containers, and to employ the resulting data to infer container wall thickness information. Once the containers have begun to cool, the thicker portions of the containers will retain heat longer than the thinner portions, and the exterior surface temperature will therefore be highest at the thicker portions of the container. Wall thickness information can therefore be inferred from the temperature profiles of the containers. However, the prior art does not disclose a technique for obtaining an absolute measurement of container wall thickness at the hot end of the manufacturing process, and it is a general object of the present invention to provide such a technique.
A method of measuring wall thickness of hollow glass articles, such as molded glass containers having interior and exterior wall surfaces, in accordance with the present invention, includes the steps of measuring intensity of electromagnetic radiation emitted by the article at a first wavelength at which intensity varies as a function of both temperature at the surfaces and wall thickness between the surfaces, and at a second wavelength at which intensity varies as a function of surface temperature at the article surface substantially independent of wall thickness between the surfaces. Since the first intensity measurement is a function of both wall thickness and temperature, while the second intensity measurement is a function solely of temperature, wall thickness between the surfaces can determined as a combined function of the first and second intensity measurements. (It will be appreciated, of course, that the term "wavelength" normally would encompass a wavelength range because sensors are not responsive solely to a specific wavelength.)
In some preferred embodiments of the invention, the first and second intensity measurements are obtained from radiation emitted from one point on the article surface. A relationship between wall thickness and surface temperature at this point on the article surface is developed from these intensity measurements. Intensity of radiation emitted from other points on the article surface can then be measured at the infrared wavelength at which intensity varies solely as a function of surface temperature, and wall thickness can be determined at such other points on the article surface as a combined function of such intensity measurements and the relationship between wall thickness and surface temperature previously developed.
In some preferred embodiments of the invention, the sensors include an area array sensor having a multiplicity of sensing elements and means for focusing onto such elements light energy (visible and/or infrared) emitted from different points on the container surface, and a second sensor responsive to energy emitted from a single point on the container surface. An absolute measurement of container wall thickness is obtained from the output of the second sensor responsive to energy emitted from the single surface point at the first wavelength, and from the output of the element on the area array sensor focused on the same point and responsive to energy emitted at the second wavelength. Given this absolute measurement of wall thickness, and thus the relationship between wall thickness and surface temperature at this one point on the container surface, wall thickness at other points on the container surface can then be determined as a function of energy indicative of exterior surface temperature incident on the other elements of the area array sensor.
In other preferred embodiments of the invention, a reflector is positioned between the two infrared sensors and the container or other article under inspection such that the detectors have fields of view that are coincident at the surface of the container. Thus, the detectors simultaneously receive radiation from a single point or area at the surface of the container to develop the associated signals representative of intensities at the first and second wavelengths. The reflector is coupled to a motor or other suitable mechanism for moving the reflector in such a way that the coincident fields of view of the detectors effectively sweep the surface of the container. In this way, the outputs of the detectors may be scanned at increments of reflector motion to obtain comparative signal data for determining wall thickness at sequential positions along the surface of the container. Most preferably, in this embodiment of the invention, the reflector is moved and the detector outputs are scanned at increments of container motion so as to obtain thickness data along the entire surface of the container. When inspecting containers that are hot from the molding process, the inspection may be performed while the containers are moved along the linear conveyor between the machine in which the molding is performed and the annealing lehr by positioning an optical inspection system on both sides of the conveyor for obtaining thickness data from both sides of the container.
The first wavelength at which intensity is measured as a function of both temperature at the container surface, and wall thickness between the surfaces, is a wavelength at which the article or container wall is substantially transparent. The second wavelength at which intensity is measured as a function of temperature at the surface, and independent of wall thickness between the surfaces, is a wavelength at which the article or container wall is substantially opaque. Transparency and opacity are, of course, relative terms. The glass composition of a container wall is substantially transparent to energy in accordance with the present invention when the transmissivity of the wall is at least 5%. The container wall is substantially opaque to energy in accordance with the present invention when transmissivity of the wall to infrared energy is less than 1%. In the preferred implementation of the invention, energy indicative of both glass temperature and wall thickness is in the visible and infrared range of 0.4 to 1.1 microns. Energy at which the wall is substantially opaque, and at which intensity therefore varies as a function of surface temperature and substantially independent of wall thickness, is preferably in the infrared range of 4.8 to 5.0 microns, and most preferably at about 5.0 microns. Standard commercially available sensors can be obtained having response characteristics within these ranges.